Digital data communications over telephone networks have traditionally been associated with data compression technologies. One reason why these technologies have been used is that this type of communications bandwidth has historically tended to be either expensive or scarce, or both expensive and scarce. Another reason is the relative unreliability of telephone networks and their tendency to introduce extraneous data, since the compression methods used with digital data often incorporates error detection and/or error correction mechanisms. Examples of data compression and error detection/correction protocols from the International Telecommunications Union (ITU) designed for use with telephone networks include the T.4 method of compressing image data, still used today in facsimile machines, and dial-up modem protocols such as V.42bis.
While the increasing use of broadband data networks has led to a diminution of the use of fixed telephone networks for data communication, there has at the same time been an increase in the use of wireless telephony for this purpose. The success of the GSM Short Message Service (SMS), with over 1000 billion text messages projected to be sent during 2005, is the most dramatic evidence of this, while the use of wireless telephones as terminals to more conventional electronic mail (email) systems is also growing fast, albeit less dramatically.
Although the description of this invention mostly discusses its applicability to email, it should be understood that it applies equally well to the transportation of any type of digital message between any type of computing device. The term computing device as used in this content should be understood to apply to any entity with a processor and memory storage which is capable of sending, receiving or processing digital messages. This includes but is not limited to personal computers of all types, network servers, terminals, mobile or fixed line telephones, pagers, and any converged devices capable of connecting to computer networks such as games machines.
In general, email messages and their attachments are treated by wireless terminals and wireless networks in the same way as they are on fixed networks, with content being transported over-the-air between the mail servers and their mobile clients “as is”. This means that when transporting messages, the entire contents of the message would normally be sent. So, for example, the common use case of someone forwarding a message and just adding the text “FYI” at the top would require the original message to be completely re-transmitted in addition to the minimal new text “FYI”.
It is recognised that this is not always an efficient use of network bandwidth, because forwarding and replying to emails very commonly leads to the development of long chains or threads.
A somewhat more efficient method of sending this type of email over wireless telephone networks has been pioneered by the Blackberry wireless email terminals manufactured by Research in Motion Ltd of Ontario, Canada (RIM). Users of Blackberry devices do not need to retransmit the original of a message received from their handsets when they want to include it in a reply, or when it is forwarded. Instead, they can send a command token with a unique message identifier corresponding to the original message as part of their new message transmitted to the Blackberry mail server. The mail server then retrieves the text of the original message and inserts the new message at the relevant point. This facility aims to minimise the bandwidth used and costs incurred by re-transmission or forwarding of emails and attachments.
The data compression technologies described above are not, however, optimised for typical use cases involving the sending and receiving of emails via scarce or expensive transport mechanisms. The cellular wireless links available to mobile telephones are currently one of the most prominent examples of such mechanisms.
Traditionally, data compression technology relies on some form of data dictionary which is transported over the data link together with the data being compressed; in the case of ITU V.42bis, this is done on a session by session basis. However, the transportation of the dictionary represents a significant overhead for low volumes of data where there is very little chance that significant data elements will be repeated a large number of times; this is the typical scenario of someone reading one or two emails on their mobile phone. As the data content changes, such dictionaries either become ineffective or else need to be updated, which can result in the consumption of still more bandwidth, yielding little or no net advantage. These problems may account for that fact that very few GSM operators have enabled V.42bis data compression on their CSD (circuit-switched data) networks.
The RIM Blackberry command token system, though capable of handling basic reply and forwarding scenarios such the “FYI” content case outlined above, does not deal very efficiently with cases where the recipient of an email modifies the text before replying or forwarding it onto another user. A familiar modification of this type is that commonly used in Unix style mail clients, where the text of an original message is included with a reply, but with each line prefixed with a special character indicating that it is a quote from the original; the usual character for this is the right-hand angle bracket“>”.
Furthermore, in the case of all such mechanisms that rely on message IDs (such as the Blackberry) the original message must always be completely sent from the server to the mobile client at least once, as otherwise there would be no meaning that could be attached to a message ID, preventing the handling of email replies or forwarding via command tokens.